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Topic: How to improve MPG's on Cummins ISL (Read 2714 times) previous topic - next topic
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How to improve MPG's on Cummins ISL

Yahoo Message Number: 77922
Hello All,

I've got an '08 CC Inspire 36' with a 425 ISL. Just passed 18,000 miles. We bought it back in June with just under 14k miles.
I am pretty religious about setting the cruise at 61 mph. Over the past summer , we did 1800 miles towing my truck and RZR (8,500 lbs total toad) and got as bad as 6.3 and as good as 7.2. I figured this was acceptable given the load I was towing.
Since then, we've had a handful of trips towing a 16' enclosed trailer, with all gear weighing 4,000 lbs max. Towing pretty much flat grades from Burbank out to the desert, and I'm getting anywhere from 6.05 - 6.60 mpg's. Again, always doing 60-61 mph. It seems to be getting worse the more I use it, regardless of if I'm towing or not! I've also had 3-4 local trips towing nothing, and still not getting over 7 mpg's.
I've read A LOT of people claiming 7.5 to high 8's in MPG's on these larger engine DP's, so what gives? What can I do to improve my chances of bettering my fuel economy? I bought this pig (lovely pig that it is) expecting to get 7.5 - 8.0 mpg's, and this low 6's is getting to me.
I'm probably due for a new air filter, and will change all the engine/tranny fluids in a few months, I changed the fuel filter 4 months ago, but what else could be wrong? Is there something that could be wrong with the programmer or something?
Tire pressure is always within spec, cold reading, 105 rear and 120 front, give or take 5 psi, but I'm certainly not low on tire pressure....

Looking for any tips on ways to combat this if possible....
Thanks

Re: How to improve MPG's on Cummins ISL

Reply #1 –
Yahoo Message Number: 77925
You might check weather you are in sixth gear at 60 MPH.
Mine just barely grabs sixth at that speed, if you are in fifth your mileage will not be good.
Where is you RPM? Cruising should probably be under 1500.
Have you tried the "MODE" button on the trany? It is supposed to help.

Bob Wexler
SOB

Re: How to improve MPG's on Cummins ISL

Reply #2 –
Yahoo Message Number: 77926
Thanks for the reply Bob.

I don't obsessively check, but every time I look over when cruising, I'm in 6th gear. I can't ever recall seeing that I was in 5th when I was cruising on the flats.
RPM is right at 1500, possibly 1550 at 61 MPH.
I typically use the Mode button, but on some short trips I have forgotten a couple of times. But the long summer trip I was always using it, and it's hasn't helped as much as I'd hope. If it took me from 6.5 to 8+, then it would have made a lasting impression, but it hasn't seemed to do a lot of help thus far....

Re: How to improve MPG's on Cummins ISL

Reply #3 –
Yahoo Message Number: 77928
I know we are at a little above you with numbers of miles at 20K. This on our 04 Allure with ISL370. We have a 40', with tags, towing a CRV. I've read many times, that the engines don't fully break in, until they units get 35k+ miles under their belts. My memory is that I'm rolling along with a 6.8 - 7.5 MPG. Higher when I'm loafing along speed wise, and lower when I'm moving out

How are you calculating the MPG? Silverleaf? Manual, both?
I would also do a test when cruising on a flat road at 61 MPH, check the RPM, and then manually down shift into 5th gear. Double check the RPM has bumped up. As mentioned, it is possible that you are still in 5th.

Weight, and pure punching the RV thru the air, does impact MPG.
You might also want to run a few tanks of a cleaner, to be sure teh injectors are clean, besides checking the air cleaner.
Other then that, if you feel you are not getting the MPG you should - then going into a Cummins facility and asking for a dyno run diagnostic, may be your next step.
Best of luck, and please report back what you determine, as we all learn...
Smitty

04 Allure 31017
04 Allure Sold to a nice new owner! Now enjoying a 07 Magna Rembrandt 45' ISX600 #6775
"We're ONDROAD for THEJRNY!" (Toad and Coach license plates, say Hi if you see us!)

Re: How to improve MPG's on Cummins ISL

Reply #4 –
Yahoo Message Number: 77929
Thanks for the reply Smitty. Some very good points, and things to look into.
I just had Kevin Waite help me out on another issue I had, and he mentioned I should list my coach # as it helps everyone help me.
Coach # is 52078.

I'm always hand calculating it. Meaning, I divide the miles displayed on the "trip" odo, and divide by gallons filled at the pump each time. I honestly wasn't aware the computer logged the MPG's.
I'll sound even dumber now, but aside from knowing there is a key port for the Silverleaf, I know nothing else about it. How do I use it? What all does it do? Assuming it's the ECM, but I don't know anything other than that...

I'll run that test with manually shifting.
So if I was still in 5th, then do I just increase driving speed until I know I'm in 6th or what?
Oh, and Kevin did mention that some people have said they gained some solid MPG's when removing the CC mud flap, so I may look into that, although I do love that purty thing :)

Thanks again everyone, keep the comments coming...

Re: How to improve MPG's on Cummins ISL

Reply #5 –
Yahoo Message Number: 77932
Bob:

My 36 ft. intrigue on this trip to FL is doing 8.4 MPG.
I had been getting much lower until I beat the temperature problems. First I must advise that My engine is an ISC 350. That said , I believe there should not be that much difference just because you have a 425 ISL.
Back to temps. Several questions.

Do you have a method of verifying how hot your engine is running other than the standard dash gauge such as Silver leaf or similar? I had a bad fan controller and the fan was running hard most of the time and that can really effect MPG. At the same time the engine stayed around 180 degrees. The engine runs much more efficiently at around 195.Mine now with correctly functioning fan controller runs from 185 to 195 and mostly closer to 195. I am not alone on this result.
Let me know

James M. Green Intrigue 11021 2000 model 36 ft. 133,000 miles plus

Re: How to improve MPG's on Cummins ISL

Reply #6 –
Yahoo Message Number: 77934
To follow up on this, after further research and discussion, I do NOT have the Silverleaf on the coach. I'm being told it'd be a good idea to purchase the Silverleaf vmspc system.
Part of me hates coming on these forums, I always end up learning of all these things I MUST have, and it costs me money! Ignorance is bliss I tell ya ;)

Re: How to improve MPG's on Cummins ISL

Reply #7 –
Yahoo Message Number: 77945
I had a 36' 1992 Dynasty with an ISC 350 and it consistantly recorded 8.5 - 9.0 mpg calculated by doing the math on each trip, and over the 27,000 miles I owned it, which equalled 8.7 mpg overall. Remember the ISC engine were smaller (8.3 liter), were free of smog stuff most of us now have, and the injection was Bosch mechanical, not federally regulated electronic control. The ISC 350 exhaust was admittedly dirty, but it was definitely more efficient and got better mileage.
Today, with a 400hp ISL in my 34' 2005 Allure, I struggle to get 7 mpg driving exactly the same way. I see maybe 9 mpg occasionally on level roads at moderate speeds on the SL, but you can't average that with stops and traffic and whatever. And note, the Silverleaf is not all that accurate so manually running the numbers turns up slightly lower numbers.
I think part of the OP's question is "has anyone experienced a signaficant improvement by taking their coach into a Cummins facility and getting the preverbal tune-up?" At least that is what I would like to know.

Stuart, #31217

Re: How to improve MPG's on Cummins ISL

Reply #8 –
Yahoo Message Number: 77947
I had a leaking lift pump which cut my fuel milage one time. Is your radiator cooling fan running in the high mode all the time. That cost me over a mpg until it was fixed. You have the very high pressure fuel injection system, I would look there first. I had a defective accumulator section for a brief period, thinning my engine oil which increased my milage! I had a defective pressure sensor on the crossover pipe from day one that sensed the boost pressure from the turbo. I also had a defective charge air cooling radiator. When they were replaced my fuel milage went up to where it is now and my horse power increased remarkably in the range from 1500 to 1900 rpm. When last checked on the dyno at CC I was putting 365 hp on the ground at 2000 rpm. That works out to be about 440 hp from a ISL 400, not bad. And yes I am one of those guys that runs at 63 +/- pulling 4500 lbs and as long as I stay away from Colorado and the Sierra Nevadas I get over 8 mpg. TWI 2004 42' Intrigue 11731. I will say it seemed like it took 40,000 miles before the unit reached its prime.

Re: How to improve MPG's on Cummins ISL

Reply #9 –
Yahoo Message Number: 77958
Have your charge air cooler checked for leaks.

Rich 2002 Magna